In radio communication systems transmitting transmission data by highly efficient frequency utilization, code division multiple access (CDMA) and orthogonal frequency division multiplexing access, schemes in which spreading code is used to multiplex code, are under active research.
Among radio communication systems are radio systems that continue to cyclically transmit from an aerial radio beacon system or from radio buoys, fixed data indicative of position and radio systems that transmit semi-fixed data indicative of temperature, etc. Among these systems, some are without an external power supply and are equipped with a battery as a main power source and a solar battery an auxiliary source.
In transmission apparatuses using a CDMA scheme or an OFDMA scheme, the peak to average power ratio (PAPR), a ratio of peak value to the average output of a code multiplexing processing unit per unit time, has gradually increased. Meanwhile, to amplify a transmission signal having a large PAPR, in the amplifier, a large backoff is set in backoff design from a 1-dB compression point and accordingly, power consumption in the amplifier also gradually increases.
On the other hand, to reduce the PAPR of a transmission signal, technology has been disclosed that staggers the phases of spreading codes according to subcarrier (see, for example, Japanese Laid-Open Patent Publication No. 2006-303552) as well as technology that induces different delay periods on transmission data groups that have been subjected to spread processing (see, for example, Japanese Laid-Open Patent Publication No. H10-178413). Further, technology has been disclosed that monitors the PAPR of a code-multiplexed transmission signal and controls the power of the transmission signal such that the PAPR becomes less than or equal to a prescribed value (see, for example, Japanese Laid-Open Patent Publication No. 2005-57582).
However, with the technology recited in Japanese Laid-Open Patent Publication No. 2006-303552, a problem arises in that when the same transmission data is code multiplexed, even if the PAPR can be reduced, when different transmission data is code multiplexed, the PAPR cannot always be reduced. In other words, depending on each combination of transmission data and spreading code, the PAPR of the transmission signal after code-multiplexing increases (see FIGS. 7 and 8). In particular, in CDMA and OFDMA, the PAPR increase is significant. Furthermore, since the phases of the spreading codes are merely adjusted, the PAPR of the code-multiplexed transmission signal cannot be changed greatly.
If the PAPR of the transmission signal becomes large and the power consumption of the amplifier increases, declines in the operation period of the apparatus and in product life will arise. Further, to cope with the PAPR of the transmission signal becoming large, a high performance amplifier is necessary, which increases cost. Even if a high performance amplifier is provided to cope with the PAPR of the transmission signal becoming large, a fixed bias corresponding to the large PAPR has to be set in the amplifier. Therefore, when the PAPR of the transmission signal is small, excess consumption current flows, resulting in wasteful power consumption.
With the technology recited in Japanese Laid-Open Patent Publication No. H10-178413 as well, depending on each combination of transmission data and delay period, the PAPR of the code-multiplexed transmission signal increases as does the power consumption of the amplifier. Further, since different delay periods are merely induced on the transmission data groups that have been subjected to spread processing, the PAPR of the code-multiplexed transmission signal cannot be changed greatly.
With the technology recited in Japanese Laid-Open Patent Publication No. 2005-57582, a problem arises in that since the power of the code-multiplexed transmission signal is controlled such that the PAPR becomes less than or equal to a prescribed value, the degree by which the power of the transmission signal is controlled varies, whereby the transmission signal degrades.